The present invention relates to the application of coating materials such as a size onto the surfaces of reinforcing fibers, and more particularly to a method and apparatus for applying metered quantities of size onto the surfaces of continuous fibers, such as glass filaments. The use of an applicator according to the present invention can provide a more uniform thin film coating of size onto the fiber surface, higher glass fiber forming efficiency and a wider coating range than prior size application technologies.
The reinforced plastic industry has historically used reinforcing fibers of glass or other materials in the form of continuous or chopped fibers, strands or rovings to reinforce polymer matrices used in making a wide range of products. For example, glass fibers have been added to thermosetting polymer matrices for use in pultrusion, filament-winding, spray-up, sheet molding and bulk molding composite-making operations.
The continuous fibers, strands and rovings used as reinforcements are made from a molten fiberizable material that is introduced into a bushing or like device. The bushing is equipped with small apertures to allow passage of thin streams of the molten material. As the molten material emerges from the bushing apertures, each stream is attenuated to form a long continuous fiber. The continuously forming fibers may be gathered into strands and wound onto spools by attaching the strands to a mechanism such as a winder or pull wheel. The rate at which the winder pulls the fiber determines its diameter. Rapid winding creates thinner fibers of smaller diameter and slower winding generates thicker fibers. The rate of attenuation also affects the movement of the filaments through the fiber-forming process, and thusly affects the amount of coating material deposited onto the surface of the moving fibers. The forming packages or doffs formed by the aforementioned winding operation are then ready for use in composite-making operations.
Several problems have been associated with the use of continuous fibers and the rovings made from these fibers. A serious problem encountered with the use of wound rovings is the breakage of the individual fibers during winding, unwinding or handling of the strands. Inter-filament abrasion of the fibers causes them to break, and as a result loose ends are separated from the fiber strands. These loose, broken ends form a roughened layer or fuzz on the surface of the fibers and causes build-up of fuzz on the contact points and other surfaces of the processing machinery. Abrasion can reduce the tensile strength of the fibers by 50% or more.
It has been recognized that, in order to reduce fuzz and to provide improved tensile strength to the fibers, it is advantageous to provide a light coating of a size material onto the surface of the fibers after they have been emitted from the fiber-forming apparatus. A size is particularly desirable when the fibers are to be used as reinforcing elements in resinous articles. More particularly, a size is desirable where the fibers are required to have specific chemical properties, such as a high compatibility with a particular resin. In addition, application of a size reduces fuzz and improves fiber processing properties such as bundle cohesion, spreadability, texture (smoothness and softness), abrasion resistance and bundle unwinding ease.
Generally, the size may be either aqueous or non-aqueous. Non-aqueous size typically includes a low-boiling organic solvent combined with other functional ingredients such as a film-forming resin, a coupling or keying agent and lubricant to form a solution. In aqueous size solutions, the resin and other ingredients are often dispersed in water to form an emulsion. The size is then solidified to provide a uniform size layer on the surfaces of the fibers.
The size may be applied to the fibers in an on-line operation immediately after the fibers are formed, or it may be applied off-line to unwound fiber strands that have been previously formed and packaged. Preferably, the fibers are wet with the size soon after they are emitted from the bushing. The application of the size at this early stage helps to protect the fibers from damage during the initial winding and handling.
In applying the size, it is important to achieve uniform coverage of all the exposed surfaces without a build-up of excess size on the production equipment, or on localized portions of the fibrous surface being treated. A variety of size applicators have been developed to meet the objective of providing a uniform coating of size onto the surfaces of the fibers.
One such method of applying size involves the use of an immersion bath, according to the dip-draw method. In this process, a bundle of fiber is pulled through an immersion bath, such as a trough, which contains the size to be applied. Excess size is removed from the fiber surfaces by passing the bundle through a narrow sizing die. One of the problems associated with this method, however, is the difficulty in obtaining a uniform coating. The degree of penetration of the sizing material within the fiber bundle is limited in this process and there is considerable clumping of the size around the orifices of the die.
Another type of applicator is a rotating roller that is continuously wetted with liquid size. In the operation of this device, drawn fibers are passed over the surface of the roller. The roller usually rotates in the same direction as the movement of the fibers, but for some applications it may counter rotate. The amount of size applied to the moving fibers is controlled by changing the flow rate of the size onto the roller, or adjusting the rate of rotation of the roller. Another type of rotating applicator uses an apron belt instead of a roller to bring the size into contact with the moving fibers.
Rotating applicators present several problems, including poor control of the amount of size that is metered to the applicator. A rotating applicator""s metering will depend on the speed of rotation of the roller, the rate of movement of the drawn fiber, the density of the fibers, and the viscosity and surface tension of the sizing material. The accuracy of the metering system relies on system variables remaining constant. However, these system variables tend to fluctuate. Minimal fluctuations in any one of these variables causes noticeable variation in the application rate, thickness and uniformity of the size. Further, rotating applicators abrade and even break glass fibers as a result of roll wrapping and fiber walking. The rotating action of these applicators also causes fibers to bundle together, thereby inhibiting the formation of a uniform size.
Spray applicators have also been used in the art as a means of applying size. In such a process, a spray of droplets of the size is directed toward a fan of fibers being formed from a bushing. Known problems associated with this process include low penetration of the size between the fibers as a result of the air-drag induced by the downward movement of the fibers. Because the relative momentum of the droplets in relation to the moving fibers is low, a majority of the spray tends to be dragged downward and does not penetrate the stream of fibers sufficiently to provide good surface coverage.
Size may be also applied using a slide applicator. In such a device, the sizing material is forced through a hole or slot at an appropriate rate to form a bead of size that is contacted with the surfaces of continuous glass fibers as they are directed past the hole or slot. The fibers are channeled past the bead using guides as they are extruded from the slot. The fibers are coated as they pass through the bead of size. A variety of die designs have been made to accommodate a variety of fiber types and size of positions. These dies are typically made of a ceramic material.
A typical defect of slide applicators is xe2x80x9cleaking flowxe2x80x9d, which is the wicking flow of liquid size between the moving fiber and the die lips. Leaking flow gives the coating bead a wavy meniscus, which causes flow rate variations in the cross-slide direction. Leaking flow also reduces the efficiency of size pick-up. One of the reasons for this leaking flow is the vortex flow generated by the high-speed fiber shearing motion relative to the diverging bead region.
Accordingly, there exists in the art a need for an applicator for various types of coatings, such as aqueous and non-aqueous size, that is capable of providing a uniform size onto the surfaces of continuously forming fibers during high-speed fiber forming operations, while eliminating commonly recognized problems such as leaking flow. There also exists a need for a process of applying size to the surfaces of reinforcing fibers, which provides a uniform thin film coating on the fiber surface, and thereby improves the efficiency of the fiber-forming process by reducing fiber breakage during formation, and which improves the size pick-up efficiency and coating operability. These needs are met by the applicator apparatus and process described below.
In accordance with a first aspect of the present invention, a slide-die applicator is provided for applying size to fibers. The slide-die applicator comprises a central body having a central surface bounded by a first edge, a second edge, a top edge and a bottom edge. The top and bottom edges lie in a common plane. The central body further comprises a rear surface and a slot. The slot has an upper lip having an upper lip surface and a lower lip having a lower lip surface. The upper lip defines an upper lip angle between the upper lip surface and the common plane. The lower lip defines a lower lip angle between the lower lip surface and the common plane. The slide-die applicator further comprises a first outwardly projecting side wall having a first inner edge attached to the first edge of the central surface. The first outwardly projecting sidewall further has a first outer edge. The slide-die applicator further comprises a second outwardly projecting side wall having a second inner edge attached to the first edge of the central surface. The second outwardly projecting sidewall further has a second outer edge. The first and second inner edges define a minimum aperture distance therebetween. The first and second outer edges define a maximum aperture distance therebetween. The first and second outwardly projecting walls define an inner region therebetween. The slot connects the central surface of the central body to a source of a size to allow the size to move from the source of the size to the central surface of the central body.
In accordance with a second aspect of the present invention, an applicator apparatus is provided for applying size to a fiber. The applicator apparatus comprises a slide-die applicator and a fluid flow control means in fluid contact with the slide-die.
In accordance with a third aspect of the present invention, a process is provided for applying one or more coating compositions onto a surface of a continuously formed fiber. The process comprises the steps of forming the fiber from a source of molten material, and passing the fiber through the applicator apparatus of the present invention.
Further objects, features and advantages of the present invention will become apparent from the detailed description that follows. It is understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention, and together with this description serve to explain the principles of the invention.